bug#50501: packages->manifest can lose the desired package output

["Ludovic Courtès" <ludo@gnu.org>]

Hi,

Liliana Marie Prikler <liliana.prikler@gmail.com> skribis:

> Am Freitag, den 10.09.2021, 12:34 +0300 schrieb Efraim Flashner:
>> I've been fighting my manifest to try to get "git" and "git:send-
>> email" both included, while using package transformations. For the
>> sake of brevity, here's a stripped down version of my manifest:
>> 
>> (packages->manifest
>>   (map specification->package+output
>>     '("git" "git:send-email")))
>> 
>> Here's what I learned from some experimentation:
>> 
>> (cmd)scheme@(guile-user)> (specification->package+output "git:send-
>> email")
>> $14 = #<package git@2.32.0 gnu/packages/version-control.scm:176
>> 7f730e918320>
>> $15 = "send-email"
>> (ins)scheme@(guile-user)> (list (specification->package+output
>> "git:send-email"))
>> $16 = (#<package git@2.32.0 gnu/packages/version-control.scm:176
>> 7f730e918320>)
>> (ins)scheme@(guile-user)> (list (specification->package+output
>> "git"))
>> $17 = (#<package git@2.32.0 gnu/packages/version-control.scm:176
>> 7f730e918320>)
>> 
>> Meanwhile, (specifications->manifest '("git" "git:send-email")) does
>> actually return a manifest with both the "out" and the "send-email"
>> outputs of git.
>> 
>> (package->manifest-entry (specification->package+output "git:send-
>> email"))
>> returns a manifest entry for "git:out" and
>> (package->manifest-entry git "send-email") returns a manifest entry
>> for
>> "git:send-email".
> I think for this use of specification->package+output you need to
> compose it with list like (compose list specification->package+output). 
> The reason you observe this behaviours is that specification-
>>package+output returns multiple values, whereas package->manifest-
> entry expects a single value which is either a package or a list of the
> form (package "output").

Yes, and Guile truncates multiple-value returns by default:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (list (values 1 2))
$7 = (1)
scheme@(guile-user)> (+ (values 1 2) 2)
$8 = 3
--8<---------------cut here---------------end--------------->8---

Ludo’.